The SI unit of pressure is the 
which is
inconveniently small for everyday use. The standard atmospheric pressure at
sea level is, by definition, 
, which is often
rounded off to 
. It equals the pressure exerted (where 
) by a 
column of mercury, or by a
m column of water. A unit of 
, called the
bar, is sometime used within SI.
Barometer readings are routinely given in mm, or, in the US, in inches (of
Hg, with 
Experimental
physicists still use the torr, which is the pressure exerted by 
of mercury. Tire pressures in the US are measured in pounds per square inch
(psi or simply "pounds''), with 
, or 
. Gauge pressure is pressure above one atmosphere, or more generally above ambient pressure. The air in a car tire inflated at 20 pounds (gauge pressure) contains air at about 35 psi, absolute pressure, and a blood pressure of 100 (gauge pressure) means 860 torr, absolute pressure.